Recent years have witnessed more and more research conducted on a light-emitting device that (i) includes, as an excitation light source, a semiconductor light emitting element such as a light emitting diode (LED) and a laser diode (LD) and that (ii) emits, as illuminating light, fluorescence generated by a wavelength converting member including a fluorescent substance when the wavelength converting member is irradiated with excitation light emitted by the excitation light source. Patent Literatures 1 and 2 each disclose an example technique related to such a wavelength converting member.
The technique of Patent Literature 1 collects ultraviolet light, emitted by a semiconductor light emitting element, with use of a light collecting lens so that the ultraviolet light falls upon a fluorescent substance having the shape of a dot with a diameter of 0.5 mm or less. This arrangement allows for optical design based on light emission from a small region, and thus facilitates optical design of a light source unit.
The technique of Patent Literature 2 produces a wavelength converting member by sintering a mixture including inorganic fluorescent substance powder and glass powder. Patent Literature 2 discloses that glass powder and inorganic fluorescent substance powder are preferably mixed at a mass ratio that falls within a range between 99.99:0.01 and 70:30.
A fluorescent substance composite member has luminous efficiency that varies according to (i) the kind and content of fluorescent substance particles dispersed in glass and (ii) the thickness of the fluorescent substance composite member. Too large an amount of a fluorescent substance causes such problems as (i) making it difficult to sinter the fluorescent substance composite member and thus leading to a large porosity, which in turn makes it difficult for excitation light to fall upon the fluorescent substance efficiently, and (ii) making it likely for the fluorescent substance composite member to have a decreased mechanical strength. Too small an amount of a fluorescent substance makes it difficult to cause the fluorescent substance composite member to emit light sufficiently. In view of this, Patent Literature 2 discloses that oxide glass powder and inorganic fluorescent substance powder are preferably mixed at a ratio within the above range.
Producing a high-luminance light source preferably involves use of a small wavelength converting member as a light-emitting section for such a light source. Using laser light as excitation light makes it possible to excite a fluorescent substance at a high light density, and thus facilitates producing a high-luminance light source.